Brpnsted acid/base

Consider first the case of adding a strong acid such as HBr to water The equation for the Brpnsted acid-base reaction that occurs between them is... 

Clearly the two reactions are analogous and demonstrate that the reaction between hydroxide ion and hydrogen bromide is simultaneously a Brpnsted acid-base reaction and a Lewis acid Lewis base reaction Br0nsted acid-base reactions constitute a sub category of Lewis acid Lewis base reactions... 

The first step of this new mechanism is exactly the same as that seen earlier for the reaction of tert butyl alcohol with hydrogen chloride—formation of an alkyloxonmm ion by proton transfer from the hydrogen halide to the alcohol Like the earlier exam pie this IS a rapid reversible Brpnsted acid-base reaction... 

The second stage is a Brpnsted acid-base reaction and is fast... 

This type of alkoxylation chemistry cannot be performed with conventional alkali metal hydroxide catalysts because the hydroxide will saponify the triglyceride ester groups under typical alkoxylation reaction conditions. Similar competitive hydrolysis occurs with alternative catalysts such as triflic acid or other Brpnsted acid/base catalysis. Efficient alkoxylation in the absence of significant side reactions requires a coordination catalyst such as the DMC catalyst zinc hexacyano-cobaltate. DMC catalysts have been under development for years [147-150], but have recently begun to gain more commercial implementation. The use of the DMC catalyst in combination with castor oil as an initiator has led to at least two lines of commercial products for the flexible foam market. Lupranol Balance 50 (BASF) and Multranol R-3524 and R-3525 (Bayer) are used for flexible slabstock foams and are produced by the direct alkoxylation of castor oil. 

A quantitative determination of the strength of Lewis acids to establish similar scales (Ho) as discussed in the case of protic (Br0nsted-type) superacids would be most useful. However, to establish such a scale is extremely difficult. Whereas the Brpnsted acid-base interaction invariably involves a proton transfer reaction that allows meaningful comparison, in the Lewis acid-base interaction, involving for example Lewis acids with widely different electronic and steric donating substituents, there is no such common denominator.25,26 Hence despite various attempts, the term strength of Lewis acid has no well-defined meaning. 

Strikingly similar results were obtained for all complexes and revealed a structure-function relationship of [M(L)(S )] complexes that can be summarized as follows Metal thioether thiolate complexes exhibit Brpnsted acid-base behavior and can be reversibly protonated. Thiolate protonation drastically influences electron densities at the metal centers and redox potentials of complexes. However, in all these reactions and electronically changing situations, the metal-sulfur core of a specific [M(S )] complex remains invariant. 

Predict the direction of Brpnsted acid/base reactions. 

However, for those that are Brpnsted acids or bases, values increase dramatically as the pH is reduced below the value(s) for the compound (Haderlein and Schwarzenbach, 1993 Broholm et al, 2001). (The pK of a Brpnsted acid/base pair is the negative log of the equilibrium constant for their interconversion through the gain or loss of a proton. As such, the pK, also represents the pH value below which the concentration of the acid exceeds that of the base, and above which the base dominates.) Schellenberg et al. (1984) introduced equations that may be used to quantify the effects of pH variations on Kp for Brpnsted acids and bases. 

T]. Values of r] for some molecules and ions are given in Table 8.4. Note that the proton, which is involved in all Brpnsted acid-base reactions, is the hardest acid listed, with p = oo (it has no ionization potential). The above equation cannot be applied to anions, because electron affinities cannot be measured for them. Instead, the assumption is made that p for an anion is the same as that for the radical Other methods are also needed to apply the treatment to polyatomic... 

In 2003, Watanabe reported the development of proton-conducting Brpnsted acid-base ILs [32], With the help of the PSE pulse sequence they could demonstrate fast proton-exchange processes between protonated imidazolium cations and imidazole. Recently, Judeinstein also published two papers on the investigation of proton-conducting ILs, here based on amines and perfluorinated acids [33,34], With a combination of diffusion NMR and varios NMR correlation methods, interionic spatial correlations have been obtained. Additionally, three different types of ion association were found for three different ILs (a) mostly associated ion-pairs, (b) dissociated ions and (c) fully dissociated protons (Fig. 4). 

We already know about step 1 of the mechanism outlined in Figure 4.7 it is an example of a Brpnsted acid-base reaction of the type discussed in Section 4.6 and formed the basis of Problems 4.9 through 4.11. 

Each of the following pairs of compounds undergoes a Brpnsted acid-base reaction for which the equilibrium lies to the right. Give the products of each reaction, and identify the acid, the base, the conjugate acid, and the conjngate base. 

Identify each of the following species as a Brpnsted acid, base, or both (a) HI, (b) CH3COO, (c) H2PO4 , (d) HSO4 . 

The subscripts 1 and 2 designate the two conjugate acid-base pairs. Thus the acetate ion (CH3C00 ) is the conjugate base of CH3COOH. Both the ionization of HCl (see Section 4.3) and the ionization of CH3COOH are examples of Brpnsted acid-base reactions. 

It is acceptable to represent the proton in aqueous solution either as or as H3O+. The formula H+ is less cumbersome in calculations involving hydrogen ion concentrations and in calculations involving equilibrium constants, whereas H30 is more useful in a discussion of Brpnsted acid-base properties. 

Consider the following Brpnsted acid-base reaction at 25°C ... 

Which is the dominant term in determining the value of AG° (and hence of the acid) (c) What processes contribute to A//° (Consider the ionization of the acids as a Brpnsted acid-base reaction.) (d) Explain why TAS° is more negative for CH3COOH. 

Sodium amide (NaNH2) reacts with water to produce sodium hydroxide and ammonia. Describe this reaction as a Brpnsted acid-base reaction. 

Many chemical reactions involve a catalyst. A very general definition of a catalyst is a substance that makes a reaction path available with a lower energy of activation. Strictly speaking, a catalyst is not consumed by the reaction, but organic chemists frequently speak of acid-catalyzed or base-catalyzed mechanisms that do lead to overall consumption of the acid or base. Better phrases under these circumstances would be acid promoted or base promoted. Catalysts can also be described as electrophilic or nucleophilic, depending on the catalyst s electronic nature. Catalysis by Lewis acids and Lewis bases can be classified as electrophilic and nucleophilic, respectively. In free-radical reactions, the initiator often plays a key role. An initiator is a substance that can easily generate radical intermediates. Radical reactions often occur by chain mechanisms, and the role of the initiator is to provide the free radicals that start the chain reaction. In this section we discuss some fundamental examples of catalysis with emphasis on proton transfer (Brpnsted acid/base) and Lewis acid catalysis. 

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